1. Field of the Invention
The present invention pertains to improvements in compositions and methods for zonal isolation for vertical, deviated, and horizontal oil and gas wells. The method involves the in-situ solidification of water based drilling muds to compressive strengths well in excess of that required for casing support, zonal isolation, and borehole stability.
2. Description of Prior Art
The general procedure of drilling an oil or gas well includes drilling a borehole using a drilling fluid. Subsequent to drilling the borehole, casing is run into the well and a cement slurry is placed in the annulus between the outside of the casing and the borehole wall. Typically, the cement slurry is placed in the annulus using a successive fluid displacement technique. In this technique, the cement is first pumped into the casing. Another fluid, often drilling fluid or water, is pumped behind the cement to displace it out the bottom of the casing and up into the annulus between the casing and borehole wall. The cement slurry is usually raised to a point above the uppermost section of the formations to be isolated and may be raised into the annulus between the casing string being cemented and a previously cemented casing.
Once placed, the purpose of the cement slurry is to harden in the annulus and provide an effective seal between the casing and the borehole. The hardened cement sheath prevents or inhibits communication of fluids between the various formations penetrated by the well. In addition to providing zonal isolation, the cement is in contact with the casing and provides mechanical support and resistance to axial movement through the shear bond developed between the interfaces of cement and pipe and cement and formation.
Bonding of the cement to the casing and borehole surfaces is critical to providing an effective seal in the annulus and for providing support for casings. Under most conditions, the bonding of cement to casing is achieved through contact of the cement particles with the surface of the casing. The resulting region of contact provides a mechanical interface which impedes movement of the casing due to high frictional forces. A fluid seal between cement and casing is effected by close contact of the cement particles at the casing surfaces which results in a region of very low effective permeability that prevents fluid migration along the interface.
Bonding between the cement and borehole wall is also achieved through contact of the cement particles with the formation or drilling fluid filter cake commonly deposited at the borehole wall during the drilling of the borehole. However, bonding between the cement and casing and borehole surfaces is not always readily achievable, especially where the drilling fluid and cement are not compatible, which is not uncommon.
Accordingly, the present invention is directed to overcoming the above noted problem in the art, and in particular to problems experienced with effective placement of casing in oil and gas wells.